Cement-less type artificial joint stem with the use of composite material

ABSTRACT

This invention provides cement-less type artificial joint stems with the use of complex material which can be connected to bone without using cement, does not get loose over a long period of time, has excellent durability, and has appropriate external form and stiffness to meet the condition of each patient. Stem  1  with the use of composite material inserted in the insertion hole  8  which is penetrated into the bone  7  and fixed to the bone  7  without using cement, has the external form of the epiphysis which fits the internal form of the insertion hole  8 , has the main part  3  with changing stiffness so that in the neighborhood of the boundary between the epiphysis and diaphysis, stiffness becomes lower as approaching toward the diaphysis, and possesses a neck part  2  to place a spherical head in an artificial joint provided at the proximal end of the main part.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to cement-less type artificial joints and inparticular relates to artificial joint stems that comprise compositematerial.

It has long been known that an artificial joint made to imitate a jointis inserted when a damaged joint is removed due to a broken bone. As oneexample of this artificial joint, FIG. 10 shows a structure of aconventional total hip prosthesis used for a hip prosthesis. This totalhip prosthesis 100 consists a socket 102 fixed to a pelvis 101, aspherical head 104 equivalent to a femoral head of a femur (103) and astem 105 embedded in the femur 103.

As shown in the figure, the socket 102 and the head 104 make a pair andhave a function of a spherical bearing. This socket 102 consists ofsynthetic resins such as high-density polyethylene, and the sphericalhead 104 comprises ceramics like zirconia or cobalt alloy. Such socket102 and the head 104 have been improved in durability with manymodifications in recent years so that they can maintain the functionslonger than life expectancy of many patients who undergo total hiparthroplasty, and the focus has been shifted from the socket 102 and thehead 104 to the stem 105 to prolong the life of the total hip prosthesis100.

The stem is often made of metal, and titanium alloy such as cobalt alloyand Ti6A1—4V is mainly used, considering the strength and effect on thehuman body.

As a method of fixing the stem to the femur, adhesive called cement-typehas been used so far, and a cement-type total hip prosthesis stem usingthe method will be described below based on FIGS. 11-15. FIG. 11 is atop view of examples of the conventional cement-type total hipprosthesis stem made of metal. FIG. 12 (A) shows the condition beforethe cement-type total hip prosthesis stem is placed, and FIG. 12(B) is asectional view of the condition after the stem is placed in the femur.FIG. 13 is a sectional view of the internal structure of the proximalside epiphysis part of the femur. FIG. 14 is an enlarged sectional viewof the internal structure of bone. Also, FIG. 15 (A) is a graph, showingthe relationship between the modulus ratio of bone and the averageporosity of bone, and FIG. 15(B) is a graph, showing the relationshipbetween the thicknesswise compression ratio of bone and the averageporosity of bone.

FIG. 11 shows various types of cement-type total hip prosthesis 105a-105 d. These external forms are generally simple with straight lines,circles and circular arcs, and there are no problems although theexternal forms of the stems 105 a-105 d are simple because the adhesiveis filled in the medullary canal constituting complex internal forms.

The method of fixing the cement-type total hip prosthesis stem to thefemur 103 will be described below based on FIG. 12. First, spongycancellous bone and bone marrow are removed from the medullary canal ofthe femur 103 with the use of a tool called broach, and an insertionhole 107 to insert the stem 105 e is formed. Next, a boneplug 108 isembedded at the bottom of the insertion hole 107, and adhesive or cement109 with two kinds of resin, base resin and hardener which are mixed atthe predetermined ratio respectively is filled in the insertion hole 107(see A). Then, the stem 105 e is inserted in the insertion hole 107 andfixed to the femur 103 as the cement 109 hardens (see B).

In the epiphysis of the femur 103 where the stem is fixed, as shown inFIG. 13, the interior is fully filled with a spongy cancellous bone 110,and the cancellous bone 110 gradually decreases as approaching from theepiphysis 112 to the lower side of the diaphysis 113, and the interiorof the diaphysis 113 is abbreviation cavities. Such bone structure ismade by the force affecting as distributed loads on the sphericalfemoral head at the tip of the epiphysis 112 and is fairly rational interms of dynamics.

Further, describing the bone structure based on FIG. 14, the outermostlayer of bone has a compact bone 111, and the compact bone 111 is a partwith high bone density and high strength. Meanwhile, the interior of thecompact bone 111 is the spongy cancellous bone 110 with more refinedcavities as approaching toward the center of bone, and the cancellousbone 110 has a weaker structure than that of the compact bone 111.

Therefore, regarding the strength characteristic of bone, as shown inFIG. 15(A) and FIG. 15(B), as the average porosity of bone (cavity ratioper unit area) increases, its modulus of elasticity and compressivestrength both decrease. For that reason, bone has a structure withdecreasing modulus of elasticity and compressive strength as approachingtoward the center away from the outer layer. As to the cement-type totalhip prosthesis stem, the stem 105 is fixed to the femur 103 byimpregnating the cement 109 within the refined cavities of thecancellous bone 110.

In this way, regarding the cement-type total hip prosthesis stem, thestem 105 is fixed to the femur 103 by hardening the cement 109, so thestem 105 can be fixed to the femur 103 for a fairly short time, whichhas an advantage in rehabilitating early for patients who undergoreplacement operation with the total hip prosthesis 100. Therefore, itis particularly effective for elderly patients who are confined to bedfor a long time and concerned with negative effects on other functionsincluding motor function.

However, the cement-type uses two kinds of resin, base resin andhardener as the cement 109, and if they are not mixed enough, or themixture ratio is inaccurate, unreacted monomer resin components whichare not polymerized would remain and have harmful effects on the humanbody through the melt-out, and it is a source of causing various damagesto the human body. Therefore, there is hesitation in using thecement-type to the youth with a long life expectancy.

Also, as to the cement-type, the stem 105 is fixed to the cancellousbone 110 of the femur 103 through the cement 109, and since thestiffness and strength of the cancellous bone 110 are not enough, theadhesive property to the stem 105 gets worse due to the weight of thestem 105, and the stem 105 gets loose or moves downward, called asinking-down phenomenon. Especially when the sinking-down phenomenonoccurs, the spherical stem 105 creates circumferential hoop stress likesevering bone. Then, when the bone is cracked, patients suffer from thepain over a long period of time since there is no way to treat it sofar.

As for the total hip prosthesis, the cement-type requires re-operationat a rate of five to twenty percent within ten years, but it isdifficult to pull the stem 105 with the cement-type out of bone, and there-operation itself is not easy.

Now, the cement-less type, fixing the stem 105 to the femur 103 withoutthe use of cement, has been developed, and the following explains theconventional cement-less total hip prosthesis stem with the use of thecement-less type, based on FIG. 16-FIG. 18. FIG. 16 is a top view of theembodiment of the conventional cement-less type total hip prosthesisstem. FIG. 17(A) is an enlarged view of the principal part of the convexportion on the side of the stem, and FIG. 17(B) is a fragmentarysectional view of the further enlarged sectional view. FIG. 18 is asectional view of the conventional cement-less type total hip prosthesisstem fixed to the femur and cut in the axial direction, which is adifferent embodiment from that of FIG. 16.

As shown in FIG. 16, the conventional cement-less total hip prosthesisstem is made of metal such as titanium alloy which is the same ascement-type, and there are various forms in stems 105 f-105 j as shownin the figure, and as to the external forms of these stems 105 f-105 j,the part below neck 115 to fix the head 104 is somewhat bigger comparedto the cement-type stems 105 a-105 e, but the forms as a whole aresimple with the use of curves between straight lines. Compared to thecement type stems 105 a-105 e, the cement-less type stems 105 f-105 jhave forms such that the gap between the external surface and internalsurface of the insertion hole 107 of the stem 105 penetrated into thefemur 103 narrows.

The cement-less type stem 105 is fixed to the femur 103, using growth ofbone within the femur 103, and the gap between the internal surface ofthe insertion hole 107 and the external surface of the stem 105 narrowsas the stem 105 is driven into the insertion hole 107 and bone growsfrom the internal surface of the insertion hole 107 toward the externalsurface of the stem 105, and thereby fixing the stem 105 to the femur103.

As to this cement-less type stem 105, there is no adverse affect on thehuman body through the melt-out of the unreacted monomer in the cement109 since the cement 109 is not used. Therefore, the cement-less typestem 105 can be also used to young patients. Moreover, in a re-operationbecause the stem 105 can be pulled out of bone with relative ease, ithelps save trouble in re-operation.

However, the cement-less type fixes the stem 105 as bone grows,narrowing the gap between the bone and the stem 105, and it takesseveral months until the bone fills the gap, and the stem 105 is firmlyfixed, and then patients need a rehabilitation period, which prolonged aperiod of patients' hospitalization, imposing a burden on patients.Moreover, due to a long period of hospitalization it was difficult toadopt the method to elderly people who were concerned with negativeeffects on other functions such as motor function.

Given this situation, in order for patients to rehabilitate, the convexportion 116 (concavity and convexity portion) is set up on the surfaceof the stem 105 so that the stem 105 can be fixed to the extent thatpatients do not have trouble living in the early stage of thepostoperative period, and the stem 105 is mechanically connected to bonewith the anchoring effect of the convex portion 116.

FIG. 17(A) and FIG. 17(B) are enlarged views of the convex portion 116for the conventional cement-less type total hip prosthesis stem, and asshown in the figures, the stem 105 can be fixed to some extent in theearly stage of patients' postoperative period as being mechanicallyconnected to bone with concavity and convexity on the surface of thestem 105 and set-in structure of minute wedges or screws between thestem 105 and the bone. The size in the concavity and convexity of theconvex portion 116 is very small, and various shapes are suggested.

Moreover, in addition to the mechanical joint, a chemical joint methodis also suggested as the convex portion 116, and for instance, crystalof hydroxyapatite, the main component of bone, is attached to thesurface of the stem 105 with adhesive or the like, and the stem 105 isfixed to the femur 103 by chemically binding hydroxyapatite of the stem105 and by growing bone. The one with either a mechanical joint orchemical joint, or both has been suggested.

In this way, by setting up the convex portion 116 on the cement-lessstem 105, the initial fixation can be achieved to some extent in theearly stage of postoperative period, which could relieve some of theburden from patients who were hospitalized for a long time.

However, in the case of the stem 105 also, it was hard to say theinitial fixation was perfect. Also, in the case of these cement-lesstype stems 105 f-105 j, the joint between the stem 105 and bone is onlypartially connected to the compact bone 111 with high bone strength andmostly connected to the cancellous bone 110 with low bone strength, andthereby the joint strength between the stem 105 and bone being weak, andthe stem 105 got loose by repetitive loads from the stem 105.

Also, the conventional stem 105 is made of metal such as cobalt alloyand titanium alloy, and because these alloys are difficult to cut, it isvery hard to process the convex portion 116 with microscopicconvexo-concave on the surface of the stem 105, which made the stem 105very expensive.

Moreover, these alloys are excellent in corrosion resistance, andbecause it is difficult to apply adhesive surface treatment to thesurface to form electrically neutral and stable oxide coating foradhesion of hydroxyapatite's crystal, the bonding strength of thehydroxyapatite is not stable and the hydroxyapatite exfoliates, which,as a result, creates a problem that the stem 105 gets loose.

Also, because the external form of the stem 105 is simple, it does notfit the internal form of the medullary canal, the load to the femur 103is concentrated, and thereby becoming a source of pain and breakdown ofbone through forcibly driving the stem 105 into the medullary canal.Regarding the elderly with weak bone strength and patients withosteoporosis, because they cannot bear such operation in which the stem105 is driven into the femur 103 with a hammer, the cement-less stems105 f-105 j could not be adopted.

In order to solve these drawbacks, a new cement-less type stem has beensuggested. FIG. 18 shows the cement-less type stem, and the stem 105 kis called custom made, and it is to provide the stem 105 k having anexternal form which fits the internal form of the medullary canal 117 inthe femur 103 of patients whom the stem 105 k is implanted.

The custom-made stem 105 k is taken pictures of each section in thetwo-dot chain line shown in FIG. 18 with a ultrasonic tomography photodevice or the like, and numerical data is made, combining these imagesin three dimensions with three dimension CAD, and the external form ofthe stem 105 k is processed based on the numerical data with anumerically-controlled processing machine (NC, CNC), and then thesurface is finished by hand.

As shown in FIG. 18, because the external form of the stem 105 k fitsthe internal form of bone, and the gap between the stem and bone issmall, the stem 105 k is fixed to bone in the early stage of thepostoperative period, which can relieve patients' burden. Also, sincethe stem can be connected with the compact bone 111 with high bonestrength, fixation of the stem 105 is strengthened, preventing the stem105 from getting loose.

However, as to the custom-made stem 105 k, as shown in the sectionperpendicular to the axis in FIG. 19, it proves that the part touchingthe internal surface of the medullary canal 117 is small in thecircumferential direction. Especially the part of the epiphysis 112 ofthe proximal side of the femur 103 touching the internal surface of themedullary canal 117 is significantly small. On the other hand, thedistal side, the contacting part is getting larger as approaching towardthe diaphysis 113. Here, the proximal side of the femur 103 means theside of the hip joint, and the distal side means the side of the kneejoint.

Although it tries to make the external form of the stem 105 k fit theinternal form of the medullary canal 117 as much as possible, theworkability of the machine work for the external form of the stem 105 kand the subsequent finish processing is required. To be more precise,generally when a three dimensional form is machined, the cutting toolfor cutting the form uses a hemispheric-tipped ball-end mill, and withthe ball-end mill, it cannot get a flat face only by the machine work,which leaves a trail like a furrow called sculpheight.

Therefore, it is necessary to smooth the surface by undercutting thesculpheight by hand after the machine work, but the metal used for thestem 105 such as titanium alloy is difficult to cut, and the finishingrequires very hard work. Therefore, the cement-less type stem made oftitanium alloy became very expensive. Moreover, when convexo-concave isformed on the stem 105 to fit the internal form of the medullary canal117, the finishing work would become more difficult, and it was toocostly to adopt the kind.

When designing the external form of the stem 105, one tries not to formconvexo-concave on the surface, ensuring that the stem 105 does not getcaught when inserting the stem in the medullary canal 117. Therefore, asshown in FIG. 19, because the internal form of the medullary canal 117is complex in the proximal side of the femur 103, the external form ofthe stem 105 k cannot correspond to the internal form, reducing the partthat contacts with the stem 105 k (see section Z1-section Z8-section inthe figure). Meanwhile, because the internal form of the medullary canalis simple in the distal side, it can easily correspond to the externalform of the stem 105 k, expanding the area that contacts with the stem105 k (see section Z9-section Z13-section in the figure).

There is a term, Fit and Fill, to describe the relationship between thestem and the medullary canal. Fit means the contact ratio to thecortical bone, which is the ratio of the length of the cortical bonetouching the stem to the entire circumference of the medullary canal ina section perpendicular to the axis of bone. Fill means the fillingratio in the medullary canal of the stem, which is the ratio of thesection area of the stem to the area of the medullary canal in a sectionperpendicular to the axis of bone.

The higher Fit and Fill is, the better the accessibility of the stem andbone and the stronger force is transmitted from the stem to the bone.Therefore, as shown in FIG. 19, because in the conventional stem 105 k,Fit and Fill is low in the proximal side of the femur 103 and Fit andFill is high in the distal side, with larger contacting area with bone,or the distal side where Fit and Fill is high receives more force comingfrom the stem 105 k to the femur 103.

As shown in FIG. 13, the ossein that constitutes the compact bone 111and the cancellous bone 110, that is the trabecular bone, is formed tocontinuously extend to a particular direction, its strength increased inthis particular direction and thus in the structure of orthotropicanisotropy. This structure is similar to that of bamboo and wooden boardof straight grain. This trabecular bone extends out from bone's externalform to the internal side in the epiphysis part 112, but in thediaphysis 113 the trabecular bone is formed along with the externalform. This means that the epiphysis 112 is strong against theperpendicular force toward the bone's surface, and the diaphysis 113,conversely, is relatively weak against the perpendicular force towardbone's surface.

From the above, for the diaphysis 113, that is the distal side, therewas a risk of a bone being destroyed when a large amount of force istransferred from the stem 105 since bone in this region is weak againstthe sidling force. Therefore, it is desirable to stabilize stem in theepiphysis (proximal side). That is, the best relationship between thestem and the medullary canal is expected in such ways that the fit andfill is high in the epiphysis section (proximal side) and the fit andfill is low in the diaphysis (distal side).

As such, it is known in the traditional system 105 that porous coatingof titanium alloy is applied on the proximal side surface of the stem105 in order to increase the conjugation of bone in the proximal side,and that fixing is not to be done in the distal side by reducing theconjugation with bone through mirror finishing the tip part of stem 105locating in the distal side. Hereafter, the fixing in the proximal sideand the fixing in the distal side are called the proximal fixing and thedistal fixing respectively.

However, as shown in FIG. 19, the fit and fill is low in the proximalside and the contacting area is small, and thus there are areas whereforce from the stem 105 is applied to bone and other areas where theforce is not applied, which results in the stress shielding. This stressshielding, deriving from bone's physiological behavior, is a phenomenonin which bone thickens in the section where force applies and,conversely, bone becomes thin in the section where force does not apply.In this way, bone becomes thin in the section where a force from thestem 105 k does not apply, reducing the conjugation with the stem 105 kand causing the stem 105 k to become loose.

Also, as shown in FIG. 19, the stem 105 k turns easily in the stem 105 kbecause the contacting area between bone and the non-circular crosssection in the proximate side—that is, the section matching the internalform of the medullary canal 117—is minimal, and because the crosssection is a near circular form in the distal side. As a result,rotation and fixation of the system 105 k had not been satisfactory.

Moreover, stainless alloy such as high corrosive resistant cobalt alloyand titanium alloy is used in the above-mentioned stem 105 k. If thehigh corrosive resistant oxide film is removed through abrasion of thesurface of the stem 105 by micro motion in the contacting area with boneresulting from the stainless alloy being embedded in the body for a longperiod of time, the micro opening called corrosion pit is generated fromthe body fluid because the salinity in the body is the same as that ofseawater. There has been a case reported, in which the metal fatigue iscaused from the corrosion pit, fracturing the stem.

As such, various materials are suggested as the stem's raw material toreplace metals. Some composite materials are among the suggestions. FIG.20 indicates the nature of the strength (fatigue strength) of thecomposite materials. First, while the fatigue strength of the titaniumalloy 118 a decreases gradually as the loading applies repeatedly, thecomposite material 119, especially in the case of the carbon fiberreinforced plastic (CFRP), has an excellent durability, in which itsfatigue strength rarely decreases even if the loading appliesrepeatedly. The symbol 118 b, shown by the dotted line in the figure,indicates the titanium alloy when it is macerated in the seawater.

For example, it has been suggested to make the center of the stemmetallic and its outer side wrapped around by the composite materialssuch as FRP (fiber reinforced plastic). In U.S. Pat. No. 4,892,552,Japanese unexamined patent publication bulletin 5-92019, and publishedJapanese translations of PCT international publication for patentapplications 6-500945, it is suggested to manifacture the stem using thecarbon fiber reinforced plastic. The stems in these proposals attain thesame stiffness as metal by using the carbon fiber reinforced plastic,and unlike metal, harmful substances do not melt out in the body bymaking the plastic that is macerating into fiber harmless to human body.

However, none of the above inventions have been in practical use in thecurrent status. That is to say, it has failed to make the center of thestem metallic and its external side wrapped around with FRP since thestem becomes loose in the early postoperative period, resulting frommicro motion between the FRP and bone or between the FRP and the centerof the metallic section. The cause of this failure is thought to be thestem's bending stiffness only applies to the center of the metallicsection, making the overall bending stiffness low, and the distributionof stress in the contacting area with bone is concentrated in the bothends, leading to the occurrence of the micro motion since the stemcannot resist to the stress.

Also, U.S. Pat. No. 4,892,552 claims that from the sheet-shaped laminatemade from carbon fiber impregnated with resin, coupons are cut out in away that the carbon fiber's direction is parallel to the external formand other coupons are cut out in a way that the carbon fiber's directionis 45°, and these two types of coupons are piled up alternately and heatand pressure are applied to it to form a bloc, and the stem ismanifactured by machining in which the bloc is scraped off. It is merelysubstituting metal with the composite material. While avoiding theharmful substance to melt out, it does not solve any other problems.

Furthermore, the unexamined patent publication bulletin 5-92019 claimsthe system having the first-direction strength support with reinforcingfiber in the longitudinal direction of the stem outside of theintermediate part that is hollow and the second-direction strengthsupport with reinforcing fiber in the 45° from the longitudinaldirection of the stem further outside. In this stem, the first-directionstrength support deals with bending stiffness and the second-directionstrength support deals with torsional stiffness with a structureutilizing the characteristics of composite material. However, thesecond-direction strength support located outside the stem ismanifactured by wrapping the strip-shaped reinforcing fiber. With thismethod it is difficult to attain the external shape that fits theinternal shape of the medullary canal, necessitating the coating layerfurther outside of the second-direction strength support, and the stemmay get loose since the stress is concentrated in the both ends of thecoating layer.

Moreover, the published Japanese translations of the PCT internationalpublication for the patent applications 6-500945 claims the systemhaving the core in the center with fiber located in the same directionas the longitudinal direction of the stem, and the filling material thatis not fiber-reinforced outside the core, and the sheath with fiberarranged spirally outside the filling material. This system also cannotprevent the stem from getting loose, similar to the above-mentionedunexamined patent publication bulletin 5-92019.

The conventional systems cited above had common problems. It was theproblem of concentration of stress caused by connecting the stem andbone. FIG. 21 explains the concentration of stress in a patterned form.FIG. 21(A) indicates the condition of stress on the adhesive joint whenthe members of the same stiffness are glued together. In this situation,the average stress applied on the adhesive joint between the member 120and the member 121 is smaller than the simplified average stresscalculated by simply dividing the compressive loading by the adhesivearea, and the stress is applied mainly on both ends of the adhesivejoints (indicated with dash line in the figure). On the other hand, thecompressive stress of the member 120 and the member 121 graduallydecreases by shear stress applied to the adhesive joint as gettingtoward the left in the figure and becomes zero at the left-end section(indicated with dashed lines in the figure).

Also FIG. 21(B) indicates the condition of stress on the adhesive jointwhen members of different stiffness are adhered. In this example, themember 121 in FIG. 21(A) is replaced by the member 122 with highstiffness. The stress is particularly concentrated at the right-endsection of the adhesive joint, and the degree of stress is greater thanthat of FIG. 21(A) (indicated with dashed lines in the figure). Also,compressive stress is drastically reduced from the right-end section ofthe adhesive joint. We know from the above that the loading istransferred intensively at the one end of the adhesive joint when onemember's stiffness is high.

Furthermore, FIG. 21(C) indicates the condition of stress on theadhesive joint when the length of adhesive joint in the example FIG.21(B) is shortened. In this case, the average stress applied to theadhesive joint increases to the extent the adhesive area becomessmaller, yet the amount of stress concentration decreases and the totalstress concentration does not change (indicated with dashed lines in thefigure). Also, while compressive stress drastically decreases from theright-end section of the adhesive joint, high stress is maintainedthrough the left-end section to the extent the adhesive section shortens(indicated with a dashed lines in the figure).

As shown in FIG. 21(A) and FIG. 21(B), we know hat the stress isconcentrated at the end points of the adhesive section. That is, thestress concentration occurs at the both ends of connecting the pointbetween the stem and bone. In particular, when comparing the stiffnessof the stem and bone, the metallic stem made from titanium alloy isequivalent to the example in FIG. 21(B) and (C) since its stiffness isgreater than that of bone, and a high loading concentration applies atthe ends of the connecting section, starting the separation of the stemand bone from this section which leads to the stem to become loose.

Given the above factors, the method in FIG. 21(D) can be considered as amethod to alleviate the occurrence of stress concentration at the endsof the adhesive joint. For the member 123, the taper section 124 isprovided on the side opposite of the adhesive joint of the member 123,varying the thickness in the half way through the connecting section. Assuch, the stiffness of the member 123 decreases on the way to theright-end section, and extended to the right-end section while keepingthe stiffness low. In this case, stress concentration drops drastically,becoming close to the average stress of the adhesive joint (indicatedwith dashed lines in the figure). Also, the distribution of compressivestress is similar to FIG. 21(C) (indicated with a dashed lines in thefigure). Making the member 123 in such a form may reduce overalladhesive stress while keeping the member's overall compressive stress.

As a result, in the example of FIG. 21(D), the stress concentration isreduced while concentrating the stress at the adhesive section otherthan the ending points, and thus the separation of the adhesive sectioncan be controlled even if the stress is concentrated.

That is, making the relationship between the stem and bone like FIG.21(D) enables the stress concentration at the diaphysis to betransferred to epiphysis, and to control the occurrence of stressshielding since a high compressive stress is maintained at the adhesivesection in its entirety. Also, the adhesive section is equivalent to thecancellous bone, and the separation of the cancellous bone from thestress concentration can be controlled at the end points of theconnecting section with the stem.

However, the conventional system is manifactured from materials that aredifficult to cut such as titanium alloy, and it was impossible toprocess in the hollow section, and thus the method in FIG. 21(D) cannotbe applied to the conventional metallic stem.

In the example in FIG. 21(D), the member's thickness is varied as ameans to change the stiffness. But for the composite material, thestiffness can also be changed by changing the direction of the compositematerial's fiber, in addition to the thickness of the member. Also, itis good to change both the thickness and the fiber's direction.

As such, considering the above situation, the invention can provide acement-less type artificial joint stem with the use of compositematerial connecting to bone without using cement, not becoming looseover a long period of time, excellent in durability, and havingappropriate external form and stiffness to each patient.

SUMMARY OF THE INVENTION

In order to solve the above issue, the cement-less type artificial jointstem with the use of composite material in the invention is structuredsuch that “the cement-less type artificial joint stem is inserted in aninsertion hole which is penetrated into bone and fixed to the bonewithout using cement, comprising: a main part which has an external formof an epiphysis approximately fitting an internal form of the insertionhole, wherein stiffness around a boundary between epiphysis anddiaphysis of the said main part varies so as to lower the stiffness asapproaching the diaphysis; and a neck to place a spherical head in anartificial joint provided at a proximal end of the main part.”

Although a specific composition of the composite material for the stemin the invention does not need to be limited, the fiber-reinforcedplastic can be used. As for the fiber, carbon fiber, ceramic fiber,glass fiber, aramid fiber can be exemplified. Turning the fiber into thecontinuous fiber, one can use it as filaments, blind shape, wovenfabrics, and nonwoven fabrics, or turning into the short fiber, one canuse it as chop shape. The carbon fibers are preferable and the highmodulus carbon fiber is the most preferable among them. As for theresin, polyether ether ketone, polyetherimide, polyether ketone,polyacryl ether ketone, polyphenylene sulfide, polysulfone can beexemplified. The most preferable is the thermoplastic resin that isharmless to the human body and does not melt out.

In terms of the method of matching the stem's external shape with theinternal shape of the insertion hole, although it is not limited to aspecific composition, one can, for example, take pictures of severalcross-sections of a patient's bone to which the stem is fixed, by usinga nondestructive tomography scanner such as CT and MRI, and generate anumerical data after converting the said cross-sectional images tothree-dimensional using three-dimensional CAD, and penetrated insertionhole with the prescribed internal shape into the patient's bone by thecomputer controlled surgical robot using the said numerical data. On theother hand, one can match the stem's external shape with the internalshape of the insertion hole by formulating the molding tool using thesame numerical data and form the external shape of the stem based on thesaid molding tool.

Furthermore, as for the method of changing the stiffness of the stem'smain part, although it is not limited to a specific composition, thestiffness can be changed, for example, by formulating the stem with thecomposite material with the prescribed thickness and making thethickness thinner as approaching from the epiphysis area to thediaphysis area. Or the stiffness can be changed by changing the fibrousdirection of the reinforced fiber included in the composite material.Also, the stiffness can be changed by reducing the reinforced fiber'sportion in the composite material, volume, or quantity as approachingfrom the epiphysis area to the diaphysis area. Moreover, the stiffnesscan be changed by reducing the elastic coefficient of the reinforcedfiber in the composite material as approaching from the epiphysis areato the diaphysis area. These methods can be used separately or incombination, and it is not limited to these examples so long as thestiffness can be changed.

According to the invention, the gap between the stem and bone can bereduced as much as possible since the stem's external shape fits theinternal shape of the insertion hole that is penetrated into bone. As aresult, the stem can be well connected to bone with the use of cement,and there is no adverse affect on the human body through the melt-out ofthe unreacted monomer from not being mixed enough or the mixture ratiois inaccurate, as in a case of cement-type stem.

Also, because the stem's external shape fits the internal shape of theinsertion hole that is penetrated into bone, the initial fixationadequate for a normal life style can be attained in the earlypostoperative period, and because the rotational anchorage is high, anearly discharge from hospital is possible through shortening thehospitalization period and an early social rehabilitation is possible,which could relieve some of the burden from patients. Also, this methodcan be utilized to the elderly, who have concerns about adverse effectsto the motor functions and other functions resulting from a long-termhospitalization.

Furthermore, because the stem's external shape fits the internal shapeof the insertion hole that is penetrated into bone, fit and fill can behigh, and the loading from the stem can be transferred to bone withoutdeviation, and the stress shielding can be controlled, and the stem notgetting loose, through weakening of the connection between the stem andbone as a result of stress shielding that makes bone skinnier, can beprevented and the stem's durability increases.

Moreover, because the stem's external shape fits the internal shape ofthe insertion hole that is penetrated into bone, the stem can be fixedwithout slamming the stem into the insertion hole with a hammer, and thestem can be utilized for osteoporosis patients and elderly people whosebone's strength is weak.

Also, because the stem's external shape in the epiphysis area fits theinternal shape of the insertion hole that is penetrated into bone, fitand fill can be high, and the stem can be fixed in the epiphysis area.That is, using an example of the femur, as the epiphysis area, the stemcan be fixed near the femur, which means the proximal fixing ispossible, transferring the loading well from the stem to bone.

Also, in the proximity of the boundary between the epiphysis area andthe diaphysis area, the stiffness of the stem's main part varies in sucha way that the stiffness becomes low as approaching toward thediaphysis. As a result, the stress concentration at the ends of theconnecting section between the stem's main part and bone can becontrolled, and the stem getting loose because of the stressconcentration that breaks away the connecting section can be prevented.Also, since the stiffness in the diaphysis area is made low, the stem'sloading is mainly transferred to the epiphysis area. If applied to thefemur, for example, the proximal fixing, in which the force istransferred in the epiphysis area that is the proximal side, can bedone.

Furthermore, the composite material is used as the stem's material, inparticular, by using the composite material that is harmless to thehuman body, there is no adverse affect to the human body unlike theconventional metallic stem in which the harmful substance to the humanbody melts out from the stem to the inside of human body. Also, thecomposite material is excellent in formability and workability comparedto the titanium alloy, and the desirable shape can easily be attained,which reduces the cost of producing the stems.

The cement-less type artificial joint stem with the use of compositematerial further comprising “a guide section, provided at the tip of themain part and placed at the disphysis, the guide section has a lowerbending and stretching/tensile stiffness than the main part.”

According to the invention, the guide section is provided in theforefront of the stem, and as a result, the stem can be easily insertedin the insertion hole during the operation when inserting the stem intothe insertion hole penetrated into bone because the stem's insertion isguided by the guide section.

Also, since the bending and tensile stiffness of the guide section ismade lower than the main part, the stress applied to the connectingsection between the guide section and bone can be less than the mainpart. To elaborate, the stem in the invention has the same compositionas the example shown in FIG. 21(D). That is, the left side of thefigure, which includes the taper section 124 of the member 123, isequivalent to the stem's main part, and the right side of it isequivalent to the guide section, the member 120 is equivalent to bone,as well as the adhesive section connecting the member 123 and member 120is equivalent to the connecting section between the stem and bone. As aresult, the stress concentration at the ends of the connecting sectionbetween the stem's main part and bone can be controlled, and may preventthe stem from getting loose due to the stem's separation from bone.Also, the stem's loading is transferred from the guide section to bonevia the main part, thus for the femur, for example, it is the proximalfixing and the stem's loading can be well transferred to bone.Furthermore, also at the guide section, the stress shielding can becontrolled for bone contacting the guide section, since the compressionstress is equally applied.

The cement-less type artificial joint stem with the use of compositematerial in the invention can also have a composition that “clearance ofa predetermined quantity between an external surface of the guidesection and an internal surface of the insertion hole is reserved.”

According to the invention, the loading is not transferred through theguide section since the clearance is formed between the insertion holeand the guide section and the guide section does not contact bone. Thatis, the fit and fill of the stem is low in the diaphysis area where theguide section is, and the stem is not fixed in this area but is fixed inthe epiphysis area where the main part is, and thus the loading from thestem can be transferred to bone as a good condition.

Also, due to bone's growth after the surgery, even if the clearancebetween bone and the guide section is filled, it is filled with thecancellous bone that has low strength, making the stress applied at theconnecting point with the guide section small. The loading from the stemis significantly applied in the epiphysis area where the main part is,and the anchorage in the epiphysis area is continuously maintained, andthe loading from the stem can be transferred to bone in a goodcondition.

The cement-less type artificial joint stem with the use of either one ofcomposite material, wherein “an external surface corresponding to theepiphysis has a convexo-concave surface treatment section thereon.” Thesurface finishing part can be a continuous convexo-concave shape, or canhave the intaglio and convexity in several places on the flat surface,or can be provided with the adhesive line that includes thehydroxyapatite. These can be used separately or in combination, and thesurface finishing part is not limited to these mentioned above.

According to the invention, the convexo-concave surface treatmentsection is provided in the external surface of the stem, and themechanical bonding strength between the internal surface of theinsertion hole and bone can be attained, and the anchorage strengthadequate for a normal life style can be attained in the earlypostoperative period. As a result, it can relieve some of the burdensfrom patients who are hospitalized for a long time, and the stem in theinvention can be utilized to elderly people.

Also, because the composite material is used for the stem in theinvention, the surface finishing part can be provided more easily thanthe conventional stem, which used titanium alloy, a material that isdifficult to be broken/cut. As a result, the stem's cost can be reducedeven with the surface finishing part.

The cement-less type artificial joint stem with the use of compositematerial, wherein “the convexo-concave external surface treatmentsection has an adhesive layer containing hydroxyapatite on the mostexternal surface, and fiber of composite material is positioned alongwith the convexo-concave external surface without breakage.” As for thehydroxyapatite, its crystal is preferable to use in order to increasethe bonding strength.

According to the invention, since the hydroxyapatite crystal is includedon the surface of the surface treatment section and the hydroxyapatitecrystal and bone are chemically bonded, the stem and bone can be gluedtogether more strongly in addition to the mechanical bonding by theconvexo-concave of the surface finishing part.

Also, since the fiber form of the composite material are continuouslyprovided inside along with the convexo-concave of the said surfacetreatment section, the fiber form of the composite material arecontinuous fibers and the strength of the composite material does notbecome low, and thus a high strength can be maintained.

Furthermore, since the composite material is used for the stem, theadhesiveness with the adhesive line, which includes hydroxyapatite, isbetter compared to the conventional stem of titanium alloy, and it isdifficult for the stem to separate from the hydroxyapatite. Also, byusing the resin for the adhesive line same as the resin used for thecomposite material, the adhesiveness becomes better between the adhesiveline and the stem.

The cement-less type artificial joint stem with the use of either one ofcomposite material, wherein “the main part comprises: a first externallayer which contacts an internal surface of the insertion hole and hasincreased torsional stiffness; a main structure layer which ispositioned inside the first external layer, continuing from the neck,and has increased bending stiffness; a core layer which is positionedinside the main structure layer and has lower stiffness than the mainstructure layer and the first external layer; and a most internal layerwhich is positioned between the core layer and the main structurelayer.”

As for the method of increasing the torsional stiffness, the torsionalstiffness can be increased by turning the direction of the compositematerial's fiber opposite of the torsional direction, for example, ±45°direction against the torsional direction. Also, as for the method ofincreasing the bending strength, the bending strength can be increasedby turning the direction of the composite material's fiber perpendicularto the bending direction.

Also, in terms of the core layer with low stiffness, resin withnon-reinforced fiber and plastic foam, or the composite material thatuses short fiber can be used, and it is not limited to a specificmaterial so long as its stiffness is lower than that of the mainstructure layer and the first external layer.

According to the invention, the main structure layer with strong bendingstiffness is provided inside the stem and the first external layer withstrong torsional stiffness is provided outside the stem. As a result,the stem's bending and torsional stiffness can be optimized.

The conventional stem was metallic such as titanium alloy, and itsstiffness was unable to change in accordance with patients' condition,and thus the stem could not be used for the patients with weak bone aswell as osteoporosis patients. However, according to the invention, thebending and torsional stiffness can be appropriately set up, and thestem can be adjusted to the characteristics of patients' bone in whichthe stem is to be filled. For example, for elderly people with weakbones and osteoporosis, the stem can be made in accordance with thestiffness of their bones. As a result, one can restrain a case in whichbone is broken due to a significant difference in the stiffness of thestem and bone, and thus the stem can be applied to patients who had beenunable to use the artificial joint.

As mentioned above, according to the invention, one can provide thecement-less type artificial joint stem with the use of compositematerial, which connects bones without using cement, not getting loosefor a long period of time, excellent in the durability, and is providedwith the stiffness and the external shape appropriate for each patient.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood bythe following detailed description of the preferred embodiments, whenconsidered in connection with the accompanying drawings, in which:

FIG. 1 (A) is a front view of the artificial joint stem with the use ofcomposite material of the invention, and (B) is the side view.

FIG. 2 (A) is the A1-A1 section view of FIG. 1, and (B) is the A2-A2section view of FIG. 1.

FIG. 3 is section views of B1-B6 in FIG. 1 which are cut in each levelperpendicular to the axes.

FIG. 4 (A) is an enlarged section view of the surface treatment section,and (B) is a further enlarged section view of the B part shown with anarrow in (A).

FIG. 5 (A) is a graph of the contact ratio to the cortical bone and thefilling ratio in the medullary canal of the stem in FIG. 1, (B) is agraph of bending and tensile stiffness, and (C) is a graph of torsionalstiffness.

FIG. 6 (A) is a front view of the other embodiment's stem of theinvention, and (B) is the side view.

FIG. 7 is section views of C1-C6 in FIG. 6 that are cut in each levelperpendicular to the axes.

FIG. 8 (A) is a graph of the filling ratio in the medullary canal of thestem in FIG. 6, and (B) is a graph of bending and tensile stiffness, and(C) is a graph of torsional stiffness.

FIG. 9 (A) is a front view of the other embodiment's stem of theinvention, and (B) is the section view.

FIG. 10 shows the structure of the conventional total hip prosthesis.

FIG. 11 is top views showing the examples of the conventional metal-madecement-type total hip prosthesis stem.

FIG. 12 (A) shows the condition before the cement-type total hipprosthesis stem is placed, and (B) is the section view, showing thecondition in which the stem is placed in the femur.

FIG. 13 is a section view of the internal structure of the epiphysis inthe proximal side of the femur.

FIG. 14 is an enlarged section view of the internal structure of bone.

FIG. 15 (A) is a graph, showing the relations between the bone's modulusratio and the average porosity of bone, and (B) is a graph, showing therelations between the thicknesswise compression ratio of bone and theaverage porosity of bone.

FIG. 16 is top views showing the examples of the conventionalcement-less type total hip prosthesis.

FIG. 17 (A) shows an enlarged view of the principal part of convexportion on the side of stem, and (B) is a fragmentary sectional view ofthe further enlarged sectional view.

FIG. 18 is a section view of the conventional cement-less type total hipprosthesis stem fixed to the femur and cut in the axial direction, whichis a different embodiment from that of FIG. 16.

FIG. 19 is section views of Z1-Z13 in FIG. 18 that are cut in each levelperpendicular to the axes.

FIG. 20 is a graph, showing the change of fatigue strength by cyclicloading of composite material and titanium alloy.

FIG. 21(A) shows the condition of stress on the adhesive joint whenmembers of the same stiffness are glued together, (B) shows thecondition of stress on the adhesive joint when members of differentstiffness are glued together, (C) shows the condition of stress on theadhesive joint when the length of the adhesive joint of the example (B)is shortened, and (D) shows the condition of stress when the stiffnessof either member is changed on the way.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, the preferred embodiments are illustrated in details based on theFIGS. 1 through 5. FIG. 1(A) is a front view of the cement-less typeartificial joint stem with the use of composite material in theinvention, and FIG. 1(B) is its side view. FIG. 2(A) is the section viewA1-A1 in FIG. 1, and FIG. 2(B) is the section view A2-A2 in FIG. 1. FIG.3 is section views of B1-B6 in FIG. 1 that are cut in each levelperpendicular to the axes. FIG. 4(A) is the section view showing theenlarged structure of the surface treatment section, FIG. 4(B) is asection view of the further enlarged B part shown with an arrow in FIG.4(A). Also, FIG. 5(A) is a graph showing the contact ratio to thecortical bone and the filling ratio in the medullary canal, and FIG. 5(B) is a graph showing the bending and the tensile stiffness, and FIG. 5(C) is a graph showing the torsional stiffness.

As shown in FIG. 1, the artificial joint stem in the example is theartificial stem for the hip joint to be fixed in the femur. The stem 1is made of the composite material and is comprised of the neck part 2,the main part 3, and the guide section 4. The neck part 2 is provided ata base end part of the stem 1, and an unshown spherical head is fixedthereon. The main part 3 is fixed on the femur, and the guide section 4is adjacent thereto.

The surface finishing part 5 is formed at the main part 3 of the stem 1,provided with concave-convex on the part of its surface. Further, asshown in the enlarged view of FIG. 4, the chemically bonded layer 6 isformed by impregnating the hydroxyapatite crystal 6 a in the plasticfilm 6 b using as the adhesive agent and bonding thereto. By theconvexo-concave of the surface finishing part 5, the mechanical bondingis made high between the stem 1 and the insertion hole 8 penetrated intobone 7 for the stem 1 to be embedded. Also, the chemical bonding withbone 7 is made high with the hydroxyapatite crystal 6 a which isimpregnated in the chemical bonding layer 6 of the surface, allowing thestem 1 to be glued together with the bone 7 more firmly. The chemicalbonding layer 6 is equivalent to the adhesive layer in the invention.

As shown in FIG. 2, the internal structure of the stem 1 is configuredto have the first external layer 9 with increased torsional stiffnesswhich contacts with the internal surface of the insertion hole 8penetrated into the bone 7, the main structure layer 10 with itsincreased bending stiffness which is placed inside the first externallayer 9 and is subsequent from the neck part 2 to the main part 3, andthe core layer 11 with lower stiffness than the main structure layer 10and the first external layer 9 that is positioned inside the mainstructure layer 10, the inner most layer 12 that is placed in betweenthe core layer 11 and the main structure layer 10, and the secondexternal layer 13, which forms the external surface of the guide section4, with lower stiffness than the structure layer 10 and the firstexternal layer 9.

The composite material used for the stem 1 is the carbon fiberreinforced plastic. As for the carbon fiber, the high modulus, highstrength carbon fiber with its elasticity of 200-650 GPa, for example,is used. Also, as for the matrix, the thermoplastic resin, such aspolyether ether ketone and polyetherimide which are harmless to thehuman body, is used. The sizing can be applied to the carbon fiber inorder to increase the bonding strength to the matrix. Incidentally, asfor the stem 1 in the example, if the carbon fiber with its elasticityof 630 GPa used and the layer with its fiber direction ±45° is formed,the layer's transverse modulus G is about 49 GPa, which has enoughstrength when comparing to the conventional titanium stem of 43.3 GPa.

For the first external layer 9 of the stem 1, the fiber form of thecomposite material are woven fabric, and the direction of the fibers isdirected ±45° to the axis of the main part 3 of the stem 1. As a result,the torsional stiffness increases and the shear loading and thetorsional loading that are applied to the stem 1 can be supported at thefirst external layer 9.

Also, for the main structure layer 10 of the stem 1, the fiber form ofthe composite material are woven fabric, and the direction of the fibersis directed toward the axis of the main part 3 of the stem 1. As aresult, the bending stiffness increases and the bending loading that isapplied to the stem 1 can be supported at the main structure layer 10.

As shown in FIG. 2 (A), this main structure layer 9 is extended from theneck part 2 to the forefront section of the main part 3. That is, it isextended to the boundary between the epiphysis area and the diaphysisarea of the bone 7, while the stem 1 is being fixed on the bone 7.Further, the core 11 goes inside of the main structure layer 10 througha given depth from the side of the guide section 4 of the stem 1.

Furthermore, the taper part 14 is formed in the internal edge of themain structure layer 10, as a result of the core layer 11 going into themain structure layer 10. The thickness of the main structure layer 10 isvaried on the taper part 14, and the stiffness of the main structurelayer 10 is changed at the taper part 14. In this case, the stiffness inthe main structure layer 10 gets lower toward the forefront side.

The core layer 11 of the stem 1 is formed with the low-stiffnessmaterial such as plastic foam, and both the inner most layer 12 and thesecond external layer 13 are made with the low-stiffness material or thelayers with its fibers directed at ±45°. The stiffness of the core layer11 and the second external layer is the minimum required stiffnessnecessary to insert the stem 1 into the insertion hole 8 in theoperation.

As for the stem 1, as shown in the section views B1-B6 in FIG. 3, theexternal shape of the stem 1 fits to the internal shape of the insertionhole 8 (the medullary canal 8 a) penetrated into the bone 7 in most ofthe cross sections perpendicular to the axis.

The production method of the stem 1 in the example is explained next.First of all, several cross-sectional images of the patients' bone, onwhich the stem 1 is fixed, are captured, by using a nondestructivetomography scanner such as CT and MRI, and a numerical data afterconverting the said cross-sectional images to three-dimensional isgenerated by using three-dimensional CAD. Then the insertion with theprescribed internal shape (the internal shape of the medullary canal ispreferable) is penetrated into the patient's bone by the computercontrolled surgical robot using the said numerical data. On the otherhand, the molding tool is manifactured using the same numerical data,and manifacture the stem 1 using the molding tool (not shown in afigure).

In formulating the stem 1, the plastic film that is impregnated withhydroxyapatite crystal is placed at the surface treatment section 5 ofthe molding tool, and the woven fabric, which is made with the carbonfiber that forms the primary surface layer 9 and the fiber made bythermoplastic resin, is laid up therein. At this time, the direction ofthe woven fabric's fibers is ±45° to the axis of the stem 1.

Furthermore, the woven fabrics, which is made with the carbon fiber andthe fiber made by the thermoplastic resin those forms the main structurelayer 10, are laid up in such a way that the fiber's direction facestoward the axis of the stem 1. The woven fabric that forms the innermost layer 12 and the second external layer 13 is laid up, and theplastic foam to be the core layer 11 is placed in the space formed bythe inner most layer 12 and the second external layer 13.

Next, close the forming die, and give heat and pressure it using theautoclave or hot plate. When doing so, the pressurization can be donefrom the inside the stem 1 as a result of the plastic foam that formsthe core layer 11 being expanded by the heat.

The convexo-concave that forms the surface treatment section 5 isengraved in the surface of the forming die of the stem 1, and thesurface finishing of the part 5 is created while the stem is made. Asshown in FIG. 4, the first external layer 9 and the main structure layer10 inside of the surface finishing of the part 5 is also formed alongwith the shape of the surface finishing 5 without its fiber cut.

As shown in FIG. 5(A), while the stem 1 processed in the way mentionedabove has the low contact ratio to the cortical bone and the fillingratio in the medullary canal, that is fit and fill, near the opening ofthe insertion hole 8, the fit and fill is higher in the more forefrontside, and undergoes the transition at about 70% contact ratio to thecortical bone and filling ratio in the medullary canal all the way tothe forefront side (side of the guide section 4).

FIG. 5(A) is the contact ratio to the cortical bone and filling ratio inthe medullary canal shown in the form of a graph (solid line), and itscontact ratio to the cortical bone and the filling ratio in themedullary canal are significantly higher than the conventionalcement-less type stem (a dashed lines) and the custom made stem (dashedlines) in which the conventional cement-less type stem is improved. Thatis, the fit and fill of the stem 1 is generally high in the main part 3and the guide section 4. The reference number 15 in the figure is thearea where the main body 3, in which the taper part 14 is not provided,is located. The reference number 16 is the area where the taper part 14of the main part 3 is provided. The reference number 17 is the areawhere the guide section 4 is located.

However, as shown in FIG. 5(B) and FIG. 5(C) of the same figure, in theepiphysis and the diaphysis area, that is the part in the main structurelayer 10 of the stem 1 where the taper part 14 is provided, the bendingand tensile stiffness are quickly decreasing and the torsional stiffnessis gradually decreasing, as getting toward the forefront side (the sideof the guide section 4) of the stem 1. As a result, because thestiffness of the guide section 4 is low although the overall fit andfill is high, and the stem's loading is transferred to the bone 7through the high-stiffness main part 3, the proximal fixing of the stem1 is possible.

This is also illustrated in FIG. 3. To elaborate, from this crosssection, in the main part 3, the main structure layer 10 is mainlyoccupied, and the bending and tensile stiffness is granted by the mainstructure layer 10 and the first external layer 9 outside of it. And thelow-stiffness core layer 11 and the internal layer 12 are expanded tothe center of the stem 1 as getting from the main part 3 to the guidesection 4, and there are only low-stiffness core layer 11 and the secondexternal layer 13 at the guide section 4. From this, we know that theloading of the stem 1 is largely transferred to bone 7 at the main part3.

The load transfer concept between the stem 1 and the bone 7 is the sameas the one shown in FIG. 21 (D), thereby the stress concentration on theboth ends of the contact layer of the bone 7 is restrained.

As such, according to the figure in this operation, because the externalshape of the stem 1 fits the internal shape of the insertion hole 8penetrated into the bone 7, the gap between the stem 1 and the bone 7can be reduced as much as possible. As a result, despite the cement-lesstype, the initial fixation adequate for a normal life style can beattained in the early postoperative period, and because the rotationalfixation is high, an early discharge from hospital is possible throughshortening the hospitalization period, and an early socialrehabilitation is possible and thus relieving the burden on the patient.Also, this method can be utilized to senior people, who have concernsabout adverse effect of motor functions and other functions resultingfrom a long-term hospitalization.

Also, because the external shape of the stem 1 fits the internal shapeof the insertion hole 8 penetrated into bone 7, the fit and fill can behigh and the loading from the stem 1 can be transferred to the bonewithout deviation, and therefore the stress shielding can be controlledand loosening the stem 1, due to weakening of the connection between thestem 1 and the bone 7 as a result of stress shielding that makes thebone 7 thinner, can be prevented, thereby increasing the artificialjoint's durability.

Furthermore, by providing the taper part 14 in the main structure layer10 of the main part 3 of the stem 1, the stiffness changes in such a waythat the stiffness becomes low as getting toward the forefront side ofthe stem 1. As a result, the stress concentration can be controlled atthe end of the contact layer between the bone 7 and the main part 3 ofthe stem 1, preventing the stem 1 from getting loose through separatingthe contact layer by the stress concentration. Also, since the stiffnessin the diaphysis area is made low, the loading from the stem 1 is mainlytransferred to the epiphysis area. That is, the proximal fixing can bedone.

Also, since the guide section 4 is provided in the forefront side of thestem 1, and the insertion of the stem 1 is guided by the guide section 4when the stem 1 is inserted into the insertion hole 8 penetrated intothe bone 7 during the operation, the stem 1 can be easily inserted intothe insertion hole 8.

Furthermore, since the surface treatment section 5 with convexo-concaveis provided on the surface of the main part 3 and the chemically jointlayer 6 having the hydroxyapatite crystal further above it, themechanical and chemical bonding between the stem 1 and the bone 7 arepossible, making it a stronger bonding, and thus the stem 1 can beprevented from getting loose.

Also, the composite material is used for the stem 1, which has bettershape formability and the workability compared to the one made of themetals, and thus the production cost of the stem 1 can be reduced.Furthermore, the surface treatment section 5 and the stem 1 are formedsimultaneously, and no additional process for providing the surfacefinishing part 5 is necessary, and thus enabling to control a risingcost even if the surface finishing part 5 is provided with the stem 1.

Next, we will describe the artificial joint stem with the use ofcomposite material with an embodiment different from the ones mentionedabove using FIGS. 6-8. FIG. 6(A) is the front view of the stem withanother embodiment of the invention, and FIG. 6(B) is its side view.FIG. 7 is section views of C1-C6 in FIG. 6 that are cut in each levelperpendicular to the axes. Also, FIG. 8(A) is the graph showing thecontact ratio to the cortical bone and the filling ratio in themedullary canal, and FIG. 8(B) is the graph showing the bending and thetensile stiffness, and FIG. 9(C) is the graph showing the torsionalstiffness. As for the parts similar to the abovementioned example, thesame signs are provided and the detailed illustration is omitted.

The stem 20 in this embodiment has a high fit and fill at the main part3, that is, in the epiphysis area, and a low fit and fill at the guidesection 4, that is, in the diaphysis area, making a perfect anchoragebetween the stem 20 and the bone 7 in the epiphysis area, that is, theproximal fixing.

As shown in FIG. 6 and FIG. 7, the taper part 21 is provided between themain part 3 and the guide section 4 for the stem 20 in this example, andthe given amount of clearance is formed between the outer surface of theguide section 4 and the internal surface of the insertion hole 8, as aresult of the external shape of the guide section 4 being smaller by thetaper part 21.

From this, as shown in FIG. 8 (A), while the contact ratio to thecortical bone and the filling ratio in the medullary canal (fit andfill) are high in the main part 3 of the stem 20, the fit and filldecreases in the taper part 21, and the fit and fill for the guidesection 4 remains low through the forefront.

As such, according to this embodiment, since the appropriate amount ofclearance is formed between the external surface of the guide section ofthe stem 20 and the internal surface of the insertion hole 8, the guidesection 4 does not contact with bone 7 in the early postoperativeperiod, thereby the loading is not transferred to bone 7 through theguide section 4.

Also, after the surgery, even if the clearance with the guide section 4is filled due to the growth of the bone 7, this part is filled with thelow density cancellous bone, and the stress applied to the joint sectionwith the guide section 4 is small, and the loading from the stem 20 islargely applied in the epiphysis area where the main part 3 is located.The anchorage in the epiphysis area is continuously maintained, and thusthe loading from the stem 20 can be transferred to the bone 7 in a goodcondition.

Furthermore, as for the stem 20 in this example, since the guide section4 is thin, the friction of the guiding 4 is low when the stem 20 isinserted into the insertion hole 8 during the surgery, and thus theinsertion can be done more easily than the stem 1 in FIG. 1.

Another embodiment of the invention using FIG. 9 will be illustratednext. FIG. 9(A) is the front view of the stem of the further embodiment,and FIG. 9(B) is its section view. The stem 30 of this embodiment hasthe characteristic of not having the guide section, and it is theembodiment of the stem 1 in FIG. 1 with the guide section 4 beingdeleted. The reference number 31 in the figure is the tertiary outerlayer, covering the bottom of the core layer 11 at the bottom of thestem 30.

For the stem 30, similar to the system mentioned above, the stem 30 canbe well fixed in the epiphysis area and provide the same effects as theone mentioned above. In this example, the core layer 11 and the tertiaryouter layer 31 can be deleted and the main part 3 can be hollow shape.

So far, we have illustrated the various embodiments of the invention,yet the invention is not limited to these embodiments, and variousimprovements as well as changes of design are possible to the extent itdoes not deviate from the scope of the invention, as indicated below.

That is, in this embodiment, the carbon fiber reinforced thermoplasticsuch as PEEK and PEI are shown as the composite materials, yet it is notlimited to these materials. For example, as for the fiber, the ceramicfiber, glass fiber, and aramid fiber can be used, and as for the ceramicfiber, the ceramic fiber having the titanium component with the siliconcarbide as a main body, such as the product name “tirano fiber” can beexemplified. Also, as for the plastic, one may use polyether etherketone, polyacryl ether ketone, polyphenylene sulfide, polysulfone, andthese raw materials can be used appropriately in combination.

Also, in this embodiment, the carbon fiber of composite material usedfor the stem 1 and the stem 20 that are same as the fibers for the mainpart 3 and the guide section 4 are shown, yet it is not limited to thesematerials. One may use the high modulus fiber for the main part 3 andthe low modulus fiber for the guide section 4, or may use the carbonfiber for the main part 3 and the low modulus glass fiber for the guidesection 4, thus these materials are not restricted so long as thestiffness of the guide section 4 is lower than that of the main part 3.

Furthermore, in this embodiment, the inner most layer 12 is providedwith the stem 1, 20 and 30, yet it is not limited to such, and the stemcan be without the inner most layer 12. As a result, one may reduce thecost of the stem since the manufacturing process of the stem is reduced.

Also, in this embodiment, in FIG. 6, the taper part 21 is providedbetween the main part 3 and the guide section for the stem 20, and theappropriate amount of clearance is formed between the external surfaceof the guide section 4 and the insertion hole 8 of the bone 7, yet it isnot limited to this. For example, the clearance between the insertionhole 8 and the guide section 4 can be the same as the clearance betweenthe main part 3 and the insertion hole 8. That is, the internal shape ofthe insertion hole 8 can be shaped along with the external shape of thestem 20. From this, too, the same effects as the one mentioned above isresulted.

The invention can provide cement-less type artificial joint stems withthe use of composite material which can be connected to bone withoutusing cement, does not become loose over a long period of time, hasexcellent durability, and has appropriate external form and stiffness toeach patient.

Also, the invention can be used not only for the total hip prosthesis ofthe femur illustrated in the embodiment, but for the implant to connectjoints such as knee joint, shoulder joint and fractured bone or for thesubstitute of damaged bone by accidents or diseases.

1. A cement-less artificial joint stem with the use of compositematerial to be inserted in an insertion hole which is penetrated intobone and fixed to bone without using cement, comprising: a main partwhich has an external form of an epiphysis approximately fitting aninternal form of the insertion hole, wherein stiffness around a boundarybetween epiphysis and diaphysis of said main part which is formed thatthe ratio of members giving stiffness on the cross section areaperpendicular to the axis decreases as approaching toward the diaphysisso as to lower the stiffness as approaching the diaphysis; and a neck toplace a spherical head in an artificial joint provided at a proximal endof the main part, and a guide section which is positioned at the tip ofthe main part in the diaphysis and has lower bending and stretchingstiffness than that of the main part, wherein the main part has thefirst external layer with heightened twisting stiffness which contactsthe internal surface of the insertion hole; the main structure layerwith heightened twisting stiffness which is positioned in the firstexternal layer and continues from the neck; the core layer with lowertwisting stiffness than the main structure layer and the first externallayer which is positioned inside the main structure layer; and the innermost layer which is positioned between the core layer and the mainstructure layer.
 2. The cement-less artificial joint stem with the useof composite material of claim 1, further comprising the main structureof the main part further comprises a taper part which thicknessdecreases as moving toward the direction of the diaphysis in theneighborhood of the boundary between the epiphysis and diaphysis.
 3. Thecement-less type artificial joint stem with the use of compositematerial of claim 1, wherein clearance of a predetermined quantitybetween an external surface of the guide section and an internal surfaceof the insertion hole is reserved.
 4. The cement-less type artificialjoint stem with the use of either one of composite material mentioned inclaims 1-3, wherein an external surface corresponding to the epiphysishas a convexo-concave surface treatment section thereon.
 5. Thecement-less type artificial joint stem with the use of compositematerial of claim 4, wherein said convexo-concave external surfacetreatment section has an adhesive layer containing hydroxyapatite on themost external surface, and fiber of composite material is positionedalong with the convexo-concave external surface without breakage. 6.(canceled)
 7. The cement-less type artificial joint stem with the use ofcomposite material of claim 2, wherein clearance of a predeterminedquantity between an external surface of the guide section and aninternal surface of the insertion hole is reserved.